Electromagnetic interference (EMI) has been a challenge of long standing with mobile platform, and particularly with aircraft electronic systems. The development of fly-by-wire control systems to reduce vehicle weight and volume increases the risk of EMI. The possible use of EMI weapons to disrupt electronic subsystems used on various forms of mobile platforms and, particularly on commercial and military aircraft, poses an additional consideration that will likely gain in importance with time.
The use of “fly-by-light” systems would eliminate the risk of EMI to various electronic systems used on mobile platforms. However, fly-by-light systems are difficult to build in a form that is both robust enough to operate in aerospace environments, and which have sufficient capability of dealing with the larger number of data and controls points in a vehicle control network implemented on a mobile platform, for example, an aircraft. Nevertheless, the use of optic technologies represents one potential way to reduce the volume and mass of the traditionally used integration and control networks implemented on mobile platforms.
A principal obstacle in implementing optics based control networks in aerospace applications has been the somewhat limited number of independent optical signals that can be transmitted per optical path (i.e., per optical fiber). Thus, a key consideration in making the use of an optical based control network practical in an aerospace application is the ability to increase the number of wavelength channels that can be implemented on each optic path. However, in aerospace applications, where various components being controlled by optical signals may be exposed to harsh environments and experience significant temperature changes, thermal drift of the wavelength bands associated with the optical channels of a given optical component must be addressed. If thermal drift could be readily compensated for, then the wavelength bands defining the independent optical channels could be placed closer together than what would be possible in a thermally uncompensated for system. This would allow a greater number of wavelength bands to be used in a given optical medium, for example, on a single optical fiber.
One approach to controlling thermal drift is employed in the telecommunications industry where presently up to 64 separate wavelength division multiplexing (WDM) channels can be put on a single optical fiber. Implementing this number of separate WDM channels requires very close control of the wavelengths that define each WDM channel. The telecommunication industry's approach to controlling thermal drift is to put all temperature sensitive devices on thermal control units which control the temperature of the devices to within about 0.1° C. Those devices are placed inside temperature control enclosures which control the temperature to within about 2.0° C. On land, the enclosures are placed in temperature control buildings. At sea, the enclosures are placed at ocean depths of known constant temperature. Obviously, this degree of temperature control is impossible and/or impractical to implement in aerospace applications. In aerospace applications, forcing large numbers of optical signals onto one fiber does not produce the overwhelming cost benefits that it does for telecommunication applications. Aerospace applications typically involve fewer signals to send, over shorter distances, and inside a vehicle. As a result, the cross complexity and mass that would be required to be added into a mobile platform, in the form of complex transmitters and receivers used to put large numbers of signals on single optical fibers, does not give aerospace applications the same cost savings that are present for telecommunication applications.
To the contrary, the requirements of aerospace applications can be met by an optical based signal in which relatively modest numbers (i.e., typically 20 or less) signals are placed on a single optical fiber. This would allow operating wavelengths to be spaced sufficiently far from each other and the wavelength bands of the various devices made sufficiently wide, such that the use of optical fibers becomes more practical in an aerospace application. Then, the temperatures of the optical devices being controlled on the mobile platform can be allowed to drift, since because of the larger spacing between bands, the signals cannot cross into each others' bands. Furthermore, if sufficiently large bandwidth channels are employed, then some signals will always pass through their designated channels, even when the bands (i.e., channels) on a transmitter and those of the other optical component receiving the optical signals, such as a router, do not accurately align.
The drawback with the above described approach is that even in a typical aerospace application in which the transmitter and receiver are located together, so that they are exposed to the same ambient temperature, the optical devices that they communicate with, such as optical routers, are typically located remotely from the receiver/transmitter. As a result, the remotely located optical routers are likely to be exposed to, and therefore operating at, different temperatures from the receiver/transmitter. In aerospace applications, this difference in temperature can be significant. The large temperature range that various, remotely located optical devices may be exposed to can cause large wavelength drifts in the input filters used with such devices. For an optical based system to work with large temperature drifts, the wavelength bands must be so wide and so widely spaced apart that only a very limited (i.e., insufficient) number of bands can be fit into a useable optical spectrum on a given optical fiber.
Thus, it would be highly desirable to provide some means for compensating for thermal drift in optical components, such as optical routers, employed on a mobile platform where the optical device can be expected to experience significantly different thermal environments from those being experienced by a transmitter/receiver that is also being carried on the mobile platform. Accurately determining the thermal drift of the wavelength bands of the optical device and compensating for the thermal drift, without the need to control the temperature of the optical device (i.e., allowing the temperature of the optical device to “float”), would allow a sufficiently large number of wavelength bands to be implemented on a given optical medium to make use of an optical based system more practical in aerospace and other applications.